C variables have to be declared

bash-output.txt

$ cat example.c 
main() {
  undeclared_variable = 42;
}

$ cc example.c 
example.c: In function ‘main’:
example.c:2:3: error: ‘undeclared_variable’ undeclared (first use in this function)
example.c:2:3: note: each undeclared identifier is reported only once for each function it appears in